M.G. Ienco

Microstructural and mechanical characterisation of some sinter hardening alloys and comparisons with heat treated PM steels

Abstract Four grades of sinter hardening materials have been compared, using industrial equipment. Three powder types were completely prealloyed; the last one was a hybrid, combining prealloying and diffusion bonding. Different amounts of Cu have been added by mixing. The lubricated mixes, containing 0.6% graphite, have been compacted at different pressures, to form gears at green densities ≥7.0 g cm−3 ; the compacts have been sintered at 1120°C, under endogas from methane and fast cooled (at least 7 K s−1 within the range 850–400°C). The final step has been stress relieving, at 180°C, for 1 h. Material properties have been investigated, focusing on porosity, pore shape, hardness, microhardness, microstructure, local chemical composition and mechanical properties. For comparison, other gears, compacted in the same tool and at the same density level, but manufactured according to a more conventional cycle, i.e. starting from less alloyed powders and adding carbonitriding, quenching and stress relieving, have been used. The analysis of the different experimental results enabled the authors to find out and outline some criteria suitable for selecting sinter hardening materials and for choosing more reliable manufacturing conditions to fulfill specific application requirements.

Nominally equivalent powders for P/M steels : Analysis of response to sintering and differences at various C content

Globalization enables P/M part makers to choose powders from different sources. Raw materials produced by a given process and having equal chemical composition are supposed to be equivalent. The differences in sintering behavior, in industrial equipment, have been investigated on P/M steels obtained from four diffusion-bonded powders (Fe + Ni + Cu + Mo) on atomized iron base, at the same alloy contents. Two levels of carbon and two sintering conditions have been investigated. Dimensional changes, C content, hardness, microhardness pattern, universal hardness, fractal analysis, pore features, microstructure features, and rupture strength have been compared, to characterize different raw materials. An index of homogeneity of microstructures, based on a specific statistical approach, does not agree completely with observed microstructures and pore feature. The results show that the claimed equivalence is not confirmed by experimental data. Analyses of microhardness and microstructure distributions seem powerful tools to rate the real equivalence of so claimed powders. P/M part makers may use the proposed approach to assess detectable differences on performances among powders that are declared as “true” substitute, so confirming or contradicting any even not negligible cost difference. The comparison methods here described and applied can also help powder producers to improve the suitability of their products to sintering processes.

Experimental Investigation of Microstructural and Mechanical Properties of New Steels for Large Blooms

In the automotive world, more and more components are produced by polymer injections using steel moulds reaching very large sizes, up to 1 m × 1 m sections or even more. These steels should have good thermal conductivity, good weldability, high machinability (especially for deep drilling), and good disposition to polishing and photoengraving. Moreover, high toughness, hardness, wear, and fatigue resistance across the whole thickness of the mould are strongly required. For satisfying these demands, chemical composition and heat treatment cycles of the steel shall be properly designed and optimized. In this paper two mould steels, which were recently developed, were experimentally investigated. Samples were machined from the surface and the core of each bloom and a full mechanical and microstructural analysis was carried out. The tensile properties at room temperature and at high temperature were investigated, together with the fracture toughness. All the obtained results were compared with the traditional and reference ISO 1.2738 mould steel. Moreover, dilatometric tests were performed to investigate the specific microstructural transformations during continuous cooling and isothermal treatments. The obtained results show good mechanical strengths for the whole thickness of the blooms associated with adequate toughness parameters. Finally, results were related to microstructural.

Mechanical Properties of Large Plastic-Mold Steel Blooms

Molds for plastic automotive components such as bumpers and dashboards are usually machined from large pre-hardened steel blocks. Due to the large size, the blooms undergo a slack quench, so that mixed microstructures occur throughout, both after quench and after the tempering stages. Mechanical properties that are both not homogeneous in the section and everywhere lower (particularly in fracture toughness) than those of correctly quenched and tempered alloy steel specimens are obtained. Successive machining to form molds may be so deep that any of the microstructure occurring at different positions in the original bloom can be found at the mold face, where notch effects are commonly present. Welding, for local shape alterations, may yield further defects.

Bainite morphologies in sintered steels: influence of materials and process conditions

ABSTRACT With the introduction of diffusion-bonded (d.b.) powders, metallographers found localised microstructure changes in sintered steels, and frequent presence of bainite. With the sinter-hardening process, the bainite fraction became a tool for rating the method. In this study, depending on different parameters, differences between bainitic structures of homogenous or not-homogeneous sintered steels have been analysed. When using pre-alloys, upper bainite is present with the same morphology. When using d.b. powders, variations of local composition lead to upper bainite of different morphology, each corresponding to a composition range. Quite high Ni local content gives rise to lower bainite, always mixed with martensite. Higher Ni contents lead to incoercible austenite. Upper bainite presents different morphologies, depending on local Ni and Cu amounts. The increase of sintering temperature leads to fewer morphologies of upper bainite, due to greater amount of diffusion processes. The lower bainite has always a typical morphology, whatever process parameters.

Metallographic techniques in gold alloy study: Delamination in a white gold alloy

Abstract The structure and phase transformations that occurred during the working and heat treatment of an eighteen-carat gold alloy used in jewel making were studied. A delamination phenomenon encountered during rolling was attributed to the formation of a discontinuous globular phase, clearly observable in the microscope, but chemically indistinguished from the matrix. It appeared that this embrittling phase was precipitated by a critical combination of deformation and intermediate annealing treatment.